1. Field of the Invention
The present invention relates to a readable and writable optical disc, and to a method for recording and a method for reproducing this optical disc. More particularly, our invention relates to an optical disc for recording multimedia data including moving picture data, still image data, and audio data, and to a method for recording and a method for reproducing this optical disc.
2. Description of the Related Art
Rewritable optical discs have for years had a maximum storage capacity of approximately 650 MB, but this has changed by the development of phase change type DVD-RAM discs with a capacity of several gigabytes. Combined with the adoption of MPEG, and particularly MPEG-2, standards for encoding digital AV data, DVD-RAM is widely anticipated as a recording and reproducing medium with applications in the AV industry, as well as the computer industry. More specifically, DVD-RAM media are expected to replace magnetic tape as the storage medium of choice for AV recordings.
Increases in the storage density of rewritable optical disc media over the last few years has made it possible to use such media for applications ranging from storing computer data and recording audio data to recording image data, including movies.
The signal recording surface of a conventional optical disc is typically formatted with lands and grooves, one of which is used as a guide groove for signal recording and reproducing. The data signal is then recorded using only the land or the groove. With the advent of the land and groove recording method, however, it became possible to record signals to both the land and groove. This development approximately doubled the storage capacity of the disc (see Japanese Unexamined Patent Application (kokai) 8-7282).
The further development of a zone CLV (constant linear velocity) method simplified and made it easy to implement a CLV recording and reproducing technique, which is an effective means of further increasing the recording density. (See Japanese Unexamined Patent Application (kokai) 7-93873).
A major topic left for future development is how to use such a potentially high capacity optical disc media to record AV data containing image data to achieve new functions and performance far surpassing conventional AV products.
With the introduction of high capacity rewritable optical disc media, optical discs are widely expected to replace conventional tape media for recording and reproducing AV content. The transition from tape to disc recording media is also expected to greatly affect both the performance and functions of AV recording and reproducing products.
One of the greatest benefits of a transition to disc is a significant improvement in random access performance. While random access to tape content is possible, it generally takes on the order of minutes to rewind a full tape. This is several orders slower than the typical seek time of optical disc media, which is on the order of at most several tens of milliseconds. Tape is therefore considered, for practical purposes, not to be a random access medium.
The random access capability of optical disc media has also made possible distributed, that is, noncontiguous, recording of AV data, which is not possible with conventional tape.
FIG. 38 is a block diagram of the drive device of a DVD recorder. As shown in FIG. 38, this DVD recorder comprises an optical pickup 11 for reading data from a disc 10, an ECC (error correction code) processor 12, track buffer 13, switch 14 for changing between track buffer input/output, encoder 15, and decoder 16. An enlarged view of a disc format 17 is also shown.
As indicated by the disc format 17, the smallest unit used for recording data to a DVD-RAM disc is a sector, which is 2 KB. Sixteen sectors are combined as one ECC block, to which the ECC processor 12 applies error correction coding.
The track buffer 13 is used for recording AV data at a variable bit rate in order to record AV data to a DVD-RAM disc more efficiently. While the read/write rate (Va) to a DVD-RAM disc is fixed, the bit rate (Vb) of the AV data is variable, based on the complexity of the AV data content (e.g., images if the AV data is video). The track buffer 13 is used to absorb this bit rate difference. This means that the track buffer 13 is unnecessary if the AV data bit rate is also fixed, as it is in the Video CD format.
This track buffer 13 can be even more effectively used by dispersed placement of the AV data on the disc. This is explained with reference to FIGS. 39(a) and (b).
FIG. 39(a) shows the disc address space. If the AV data recorded is divided between contiguous area A1 between addresses a1 and a2, and contiguous area A2 between a3 and a4, as shown in FIG. 39(a), the AV data can be continuously reproduced by supplying data accumulated in the track buffer 13 to the decoder while the optical head seeks from a2 to a3. This is shown in FIG. 39(b).
Once reading AV data starts from a1 at time t1, it is both input to the track buffer 13 and output from the track buffer 13 with data accumulating in the track buffer at the rate (Vaxe2x88x92Vb), that is, the difference between the input rate Va to the track buffer and the output rate Vb from the track buffer. This continues to address a2 at time t2. Assuming that the data volume accumulated to the track buffer at this time is B(t2), data supply to the decoder can continue until the data B(t2) accumulated to the track buffer is depleted at time t3 at which reading resumes from address a3.
In other words, if it is assured that a certain volume of data ([a1, a2]) is read before a seek operation is performed, AV data can be continuously supplied to the decoder while the seek is in progress.
It should be noted that this example considers reading, that is, reproducing, data from DVD-RAM, but the same concept applies for writing or recording data to DVD-RAM.
It will thus be obvious that insofar as a specified amount of data is recorded continuously to a DVD-RAM disc, continuous reproduction and recording is possible even if the AV data is noncontiguously recorded to the disc.
A common AV data format is described next below.
As noted above, AV data is recorded to DVD-RAM media using the MPEG international standard, also known as ISO/IEC 13818.
Even though DVD-RAM discs have a large, plural gigabyte, capacity, this is still not sufficient for recording uncompressed digital AV data. A way to compress and record AV data is therefore necessary. MPEG (ISO/IEC 13818) is now widely used around the world for AV data compression. MPEG decoders (compression/decompression ICs) have also been realized with advances in IC devices. This has enabled the DVD recorder to handle MPEG compression and decompression internally.
MPEG signal processing is able to achieve high efficiency data compression chiefly as a result of the following two features.
First is that compression using a time correlation characteristic between frames (known as pictures in MPEG) is used in conjunction with conventional compression using a spatial frequency characteristic for moving picture data compression. Each video sequence of an MPEG video signal stream is divided into one or more groups of pictures, each group of pictures comprising one or more pictures of three different types: I-pictures (intraframe coded pictures), P-pictures (predictive-coded pictures, that is, intracoded with reference to a preceding picture), and B-pictures (bidirectionally predictive-coded pictures, that is, intraframe coded with reference to preceding and following pictures).
FIG. 40 shows the relationship between I, P, and B pictures. As shown in FIG. 40, P-pictures refer to temporally preceding I- or P-pictures in the sequence, while B-pictures refer to the first preceding and following I- or P-pictures. It should also be noted that because B-pictures reference an upcoming I- or P-picture, the display order of the pictures may not match the coding order of the pictures in the compressed data bitstream.
The second feature of MPEG coding is that code size is dynamically allocated by picture unit according to the complexity of the image. An MPEG decoder has an input buffer, and by accumulating data in this decoder buffer, a large amount of code can be allocated to complex images that are difficult to compress.
Three types of audio coding are used for the audio portion of a DVD-RAM recording: MPEG audio with data compression; Dolby Digital(copyright) (also known as AC-3); and noncompressive linear pulse code modulation (LPCM). Both Dolby Digital(copyright) and LPCM are fixed bit rate coding methods, but MPEG audio coding can select from several compression rates on an audio frame basis, although audio compression is not as high as video stream compression.
The resulting compressed video and audio streams are multiplexed to a single stream using a method known as the MPEG system. FIG. 41 shows the organization of an MPEG system stream. As shown in FIG. 41, each 2 KB sector comprises a pack header 41, packet header 42, and payload 43. The MPEG system thus has a hierarchical structure comprising packs and packets. Each packet comprises a packet header 42 and payload 43. AV data is segmented from the beginning into blocks of an appropriate size for storage to the payload 43.
Referring to the AV data stored in the associated payload 43, the packet header 42 contains a stream ID for identifying the data stored in the associated packet, and a decoding time stamp (DTS) and presentation time stamp (PTS) identifying the decoding time and presentation time of the data contained in the payload in 90 kHz precision. If the decoding and presentation are simultaneous, as in the case of audio data, the DTS can be omitted.
A pack is a unit of plural packets. In DVD-RAM, however, there is one pack for each packet, and each pack therefore comprises a pack header 41 and packet (containing a packet header 42 and payload 43).
The pack header contains a system clock reference (SCR) expressing with 27 MHz precision the time at which the data contained in this pack is input to the decoder buffer.
An MPEG system stream thus comprised is recorded one pack to a sector (=2048 bytes) on DVD-RAM. A decoder for decoding the above-noted MPEG system stream is described next below. FIG. 42 is a block diagram of an exemplary decoder model (P_STD) of an MPEG system stream decoder. Shown in FIG. 42 are the system time clock (STC) 51, that is, the internal reference clock for decoder operation; a demultiplexer 52 for decoding (demultiplexing) the system stream; video decoder input buffer (video buffer) 53; video decoder 54; re-ordering buffer 55 for temporarily storing I and P pictures for absorbing the difference in the coding (data) sequence and presentation sequence that occurs between B pictures and I and P pictures; a switch 56 for adjusting the output order of the I, P, and B pictures buffered in the re-ordering buffer 55; an audio decoder 58; and audio decoder input buffer (audio buffer) 57.
This MPEG system decoder processes the above-noted MPEG system stream as follows.
When the time indicated by the STC 51 and the SCR written to the pack header match, the pack is input to the demultiplexer 52. The demultiplexer 52 then interprets the stream ID in the packet header, and passes the audio stream and video stream contained in the payload data to the appropriate decoder buffers. The PTS and DTS are also read from the packet header.
When the times indicated by the STC 51 and DTS match, the video decoder 54 reads and decodes the picture data from the video buffer 53. I and P pictures are stored to the re-ordering buffer 55 while B pictures are presented directly to screen. If the picture being decoded by the video decoder 54 is an I or P picture, the switch 56 switches to the re-ordering buffer 55 to output the previous I or P picture from the re-ordering buffer 55; if a B picture is decoded, the switch 56 switches to the video decoder 54.
Similar to the video decoder 54, the audio decoder 58 reads and decodes one audio frame of data from the audio buffer 57 when the PTS matches the STC 51 (a DTS is not recorded for audio data).
An exemplary method of multiplexing an MPEG system stream is described next with reference to FIGS. 43(a) to (d). Note that a sequence of video frames is shown in FIG. 43(a), the change in data storage to the video buffer is shown in FIG. 43(b), a typical MPEG system stream is shown in FIG. 43(c), and an audio signal is shown in FIG. 43(d). Each of FIGS. 43(a) to (d) are shown on a common time base (horizontal axis). The vertical axis in FIG. 43(b) indicates the amount of data stored to the video buffer. The bold line in this graph thus indicates the change over time in the buffered video data volume. The slope of this line is indicative of the video bit rate, and shows that data is input to the video buffer at a constant rate. The decrease in buffered data at regular intervals indicates the progression of data decoding. The intersection of the dotted line extension of the graphed line with the time base (horizontal axis) indicates the time at which video frame transfer to the video buffer begins.
MPEG encoding is described next using by way of example coding a complex image A in the video data stream. As shown in FIG. 43(b), image A requires a large coding block, and data transfer to the video buffer must therefore begin from a time t1 before the image A decoding time. Note that the time from data input start time t1 to decoding is referred to as vbv_delay below. AV data is thus multiplexed to the position (time) of the shaded video pack.
Unlike video data, audio data does not require dynamic coding size control. It is therefore not necessary for audio data transfer to start before decoding starts, and audio data is thus typically multiplexed only slightly before decoding starts. Video data is thus multiplexed to the MPEG system stream before the audio data.
It should be further noted that data can be accumulated to the buffer for a limited time in the MPEG system. More specifically, the MPEG system standard requires all data other than still image data be output to the decoder from the buffer within one second of being stored to the buffer. This means that there is at most a one second offset between video data and audio data multiplexing (or more precisely, the time required for video frame reordering).
It will also be obvious that while the MPEG system stream is described above with video data preceding the audio, the audio can theoretically precede the video. This type of stream can be purposely generated by using for the video data simple images to which a high compression rate can be applied, and transferring the audio data earlier than required. Even in this case, however, the audio can precede the video by at most one second due to the restrictions imposed by the MPEG standard.
Video CD
Video CD, a moving picture format incorporating an entry point concept for playback control, is described next.
The Video CD standard was published in 1993. Version 2.0 of the standard, incorporating a playback control feature, was released the following year in 1994. The video CD can store a maximum 74 minutes of video compressed using the MPEG-1 standard, together with a maximum of 2000 high resolution still images (704xc3x97480 dots). A simple menu is compiled by the playback control function so that the presented content can be controlled to display only a required segment or so that a user can select specific content for display.
The Video CD format allows recording of absolute addresses on the disc as xe2x80x9centry points.xe2x80x9d An entry point is a specific address where the playback path can be entered to begin playback. Entry points can be achieved by using address information and time information. By using entry points, it is possible for playback to jump to a specifically recorded absolute address when playback reaches an entry point in the playback path, and the disc player can thus be controlled to jump from point to point in the disc content.
The Video CD format, however, requires a 1:1 correlation between entry points and the bitstream, and cannot use independent entry points to a plurality of reproduction paths.
In addition, Video CD is a non-writable medium, which means that the user cannot add or delete entry points to the content. The user therefore cannot create a reproduction path or entry points with logical meaning, and cannot make effective use of the disc""s random accessibility.
Digital Video
Digital video tape, and particularly the Digital Video Cassette Tape Recorder (DVC) medium that has become most popular, is described next.
Published in 1994, the DVC standard records and reproduces at 19 Mb/s to 30 Mb/s using discrete cosine transform (DCT) and variable length coding (VLC) for image compression and signal processing.
The subcode data recorded with the video data can include, in the DVC format, the track number (Title Time Code) indicative of the recording time from the first recorded frame at the beginning of the tape, a time code (Rec Date) indicating the date the recording was made, and a time code (Rec Time) indicating the time the recording was made. This makes it possible to detect interruptions in the time code and use these interruptions as entry points to the content.
The drawback to this scheme is that because management information such as is used by a computer is not present, it is not possible to freely set a desired time as an entry point from which playback is possible.
Another obvious drawback to DVC is it is a tape-based medium. Random access performance is thus poor, and plural reproduction paths cannot be achieved.
To use plural reproduction paths or selectable entry points with DVC type media, the reproduction device must have memory to store this data, and the data cannot be used on different reproduction devices.
The introduction of DVD-RAM media solves the problem of random access performance present in DVC media, and makes it possible to achieve a new consumer AV product whereby entry points to plural reproduction paths on a Video CD can be freely used.
The object of the present invention is to provide a DVD recorder that solves following problems including the hindrance of obtaining maximum performance from DVD-RAM media, a high capacity rewritable storage medium widely anticipated as the next generation in AV recording media.
That is, the greatest problem in recording entry points to a plurality of reproduction paths and using these entry points for reproduction on a DVD recorder is how to best utilize the unique random accessability of the disc medium to set individual entry points as desired to any of a plurality of reproduction paths, thus achieving functionality that is not possible with tape media.
To resolve the above problem, in an optical disc for recording a program stream of audio-visual content and management information for managing the program stream, the management information preferably comprises, according to the present invention: information (C_V_S_PTM) for specifying a start time of the program stream; information (C_V_E_PTM) for specifying an end time of the program stream; and entry point information (M_C_EPI/S_C_EPI) for accessing the program stream at a desired point, and reproducing the program stream from the desired point.
It is therefore not necessary with an optical disc according to the present invention for entry point information to be incorporated into the program stream itself.
When the program stream is moving picture content, the entry point information is preferably time information (EP_PTM). Using time information makes it possible to determine the distance (time) from the beginning of the reproduction path.
When the program stream is still image content, the entry point information is preferably still image number information (S_VOB_ENTN). This still image number information represents the number of the still image in the bitstream, and therefore, makes it possible to determine how far from the beginning of the reproduction path.
Yet further preferably, the entry point information also includes text information (PRM_TXTI). By also including text information, the content of a desired access point can be displayed.
In a further optical disc for recording a program stream of audio-visual content, and management information for managing the program stream, the management information contains, according to the present invention: information (C_V_S_PTM) for specifying a first start time of the program stream; information (C_V_E_PTM) for specifying a first end time of the program stream; and first entry point information (M_C_EPI/S_C_EPI) for accessing the program stream at a desired point and reproducing the program stream from the desired point when reproducing the program stream from the first start time to the first end time; information (C_V_S_PTM) for specifying a second start time of the program stream; information (C_V_E_PTM) for specifying a second end time of the program stream; and second entry point information (M_C_EPI/S_C_EPI) for accessing the program stream at a desired point and reproducing the program stream from the desired point when reproducing the program stream from the second start time to the second end time.
There are cases when the program stream segment defined as a first reproduction path from the first start time to the first end time, and the program stream segment defined as a second reproduction path from the second start time to the second end time, overlap. However, even if these first and second reproduction paths overlap, entry point information can be individually and separately set for both paths. Entry points set for the first reproduction path therefore do not work on the second reproduction path.
The present invention further relates to an optical disc player for reproducing an optical disc according to the present invention as described above. The optical disc player comprises: a storage means (7802) for reading and storing entry point information from the optical disc; a decoder (7806) for decoding the program stream and generating address information during program stream reproduction; a conversion means (7802) for converting the address information to point information in the program stream; a selection means (7802) for selecting entry point information closest to the point information; a conversion means (7802) for converting the selected entry point information to address information; and a drive means (7808) for jumping to a location based on the converted address information. The decoder decodes and reproduces from the jump destination.
In the optical disc player, the point information is preferably time information (EP_PTM) when the program stream is moving picture content.
Further preferably, when the program stream is still image content the point information is still image number information (S_VOB_ENTN).
Yet further preferably, the entry point information further includes text information (PRM_TXTI), and the decoder further reproduces the text information.
The present invention can be further expressed as a playback method for playing back an optical disc where the optical disc is an optical disc according to the present invention as described above. This playback method includes reading and storing entry point information from the optical disc; decoding the program stream and generating address information during program stream reproduction; converting the address information to point information in the program stream; selecting entry point information closest to the point information; converting the selected entry point information to address information;jumping to a location based on the converted address information; and decoding and reproducing from the jump destination.
The present invention also provides an optical disc recorder for recording to an optical disc according to the present invention as described above. The optical disc recorder comprises: an interface (7801) for receiving entry point information input; means (7804, 7806) for generating address information at the time the entry point information is received; a conversion means (7802) for converting the address information to entry point information in the program stream; a storage means (7802) for temporarily storing the entry point information; and a drive means (7808) for recording the stored entry point information to the optical disc.
As noted above, when the program stream is moving picture content, the point information of this optical disc recorder is preferably time information (EP_PTM).
Yet further preferably, the point information is still image number information (S_VOB_ENTN) when the program stream is still image content.
Yet further preferably, the entry point information further includes text information (PRM_TXTI), and the storage means generates and stores the text information.
The invention also provides a recording method for an optical disc according to the present invention as described above. This recording method comprises receiving entry point information input; generating address information at the time the entry point information is received; converting the address information to entry point information in the program stream; temporarily storing the entry point information; and recording the stored entry point information to the optical disc.